Verapamil, an investigational drug useful in cardiovascular disease, has been shown in animals to be beneficial following myocardial infarction. We investigated the effects of stepwise incremental doses of verapamil on heart rate, blood pressure and myocardial collateral blood flow in dogs. Blood flow to normal heart tissue increased with low doses of verapamil that did not change heart rate or blood pressure. Higher doses of verapamil increased normal tissue flow further, but also produced an increase in heart rate without blood pressure changes. Blood flow to the ischemic myocardium was not altered by any dose of verapamil tested.